Topic outline
- Origin of magnetism.
- Properties of magnets.
- Magnetic and non-magnetic materials.
- The basic law of magnetism.
- Making magnets (magnetisation).
- Hard and soft magnetic materials.
- Demagnetisation.
- Domain theory of magnetism.
- Storage and uses of magnets.
- Magnetic field patterns.
- The earth’s magnetic field.
- Project: Construction of a simple magnetic compass.
Origin of magnetism
The people of Magnesia in Asia Minor observed that certain kinds of naturally occurring ores possessed an iron-attracting property. The ore was discovered near the city of Magnesia and hence it was named Magnetite. Huge lumps of magnetite were often called lodestone meaning “leading” stone or natural magnet. Chemically lodestone consists of iron oxide.
Properties of magnets
(a) Magnetic poles
Experiment 1.1: To determine the magnetic poles
Dip a bar magnet into a container of iron filings. Remove the magnet from the container and record what you observe. It is observed that most iron filings remain clustered around the ends of the magnet as shown in Fig. 1.1.
The ends of a magnet where the attraction is strongest are known as the magnetic poles. A straight line drawn passing through these ends is called the magnetic axis of the magnet (Fig. 1.2).
(b) Directional property of a magnet
Experiment 1.2 : To establish the directional property of a magnet
Suspend a bar magnet freely at its centre by a length of a cotton thread from a support. Make sure there are no steel or iron objects near the magnet. Displace the magnet slightly so that it swings in a horizontal plane. Note the direction in which the magnet finally comes to rest. Repeat the experiment at different places.
It is found that the bar magnet rests in a north-south (N-S) direction of the earth. The pole that points towards the north is called the north seeking pole or simply the north pole (N). The other pole is called the south seeking pole or south pole (S). The magnet comes to rest with its axis in a vertical plane called the magnetic meridian (Fig. 1.3 (b)).
Identification of the poles of a magnet
In order to identify the poles of a magnet, the ends are usually painted in different colours. For example, the N-pole is painted red while the S-pole is painted white or blue. In other cases the whole bar is painted red with a white dot or spot on one end to identify the north pole.
Types of magnets
Magnets are named according to their shapes, as shown in Fig. 1.4.
(a)
(b)
(c)
(d)
Fig. 1.4: Types of magnets.
Another type of magnet is the ceramic or magnadur magnet. This has its poles at its faces (Fig. 1.5). These types of magnets are stronger than other magnets of comparable size.
Magnetic and non-magnetic materials
Materials may be classified according to their magnetic properties. There are those that are attracted by magnets and others that are not attracted.
Experiment 1.3: To determine magnetic and non-magnetic materials
Place some iron nails on the bench. Bring a bar magnet close to the iron nails and record what happens. Repeat the experiment with other materials e.g. copper, cobalt, steel, sulphur, brass, wood, cork, nickel, plastic, wax, glass, carbon, aluminium, paper, chalk etc. Record your observations in a tabular form shown in Table 1.1.
Table 1.1
Substances attracted by a bar magnet | Substances not attracted by a bar magnet |
1. | 1. |
2. | 2. |
3. | 3. |
4. | 4. |
Some materials are attracted while others are not attracted by bar magnets. Those which are attracted are called magnetic materials while those which are not attracted are called non-magnetic materials. Examples of non-magnetic materials include copper, brass, aluminium, wood, cork, plastic, etc. The magnetic materials that are strongly attracted by a magnet are called ferro-magnetic materials. These include nickel, iron, cobalt and steel.
The basic law of magnetism
Experiment 1.4: To investigate the action of magnets on each other
Suspend a bar magnet using a cotton thread with its north and south pole clearly marked. Bring a S-pole of a second bar magnet slowly towards the S-pole of the suspended magnet. Observe what happens (Fig. 1.6 (a)). Repeat the experiment using the S-pole of the suspended magnet and the N-pole of the second magnet (Fig. 1.6 (b)). Repeat using the other poles and record your observation in a table (See Table 1.2).
Table 1.2
Pole of suspended magnet | Pole of second magnet | Observation |
South | South | |
South | North | |
North | South | |
North | North |
It is observed that a north pole attracts a south pole, a north pole repels a north pole and a south pole repels a south pole. These observations are summarised as:
Unlike poles attract while like poles repel. This is called the basic law of magnetism or the first law of magnetism.
Testing polarity of magnets
Experiment 1.5: Test for polarity of a magnet
Freely suspend a bar magnet as shown in Fig. 1.7. Bring the two poles of the magnet in turn close to a nail placed on a table. Record your observations. Repeat the procedure, using a second bar magnet instead of the nail.
We observe that there is attraction when the south and north pole of the suspended magnet is brought near the nail. When the second bar magnet is used, both attraction and repulsion are observed. We can conclude that there is always an attraction between a magnet and a magnetic material or between the unlike pole of another magnet. But there is always a repulsion between two like poles of a magnet. Repulsion is therefore, the only sure way of testing for polarity of a magnet. The poles of the suspended magnet must be known.
Exercise 1.1
- State two properties of a magnet.
- State the basic law of magnetism.
- Explain what would happen to a U-shaped magnet if it is freely suspended as shown in Fig. 1.8.
- Name four types of magnets.
- Explain the terms:
- south pole of a magnet.
- north pole of a magnet.
- Describe an experiment to explain the existence of magnetic poles.
- Explain the meaning of the following terms:
- a magnet.
- a magnetic substance/material.
- a non-magnetic material.
- a ferro-magnetic material.
- What is the main difference between a ceramic magnet and a bar magnet?
- How would you identify the polarity of a magnet whose poles are not marked?
- Group the following materials into magnetic and non-magnetic materials. Zinc, paper, aluminium, graphite, steel and plastic.
- You have been provided with the following;
- a rod labelled M, which is a magnet.
- a rod labelled S, which is a magnetic material.
- a rod labelled N, which is a non-magnetic material.
Explain how you would identify them.
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